The main objectives of the project were the genetic improvement of maize for a more successful incorporation of maize into cropping systems on acid soils of the tropics and the evaluation of the prospects and limitation of such a genetic approach compared to an agronomic approach based on the amelioration of soil acidity using lime, fertiliser, and organic amendments.
The soils from three of the field experimental sites used in the breeding programmes and for the agronomic trials have been carefully characterised using appropriate methodologies by CIRAD. The analysis revealed that the soils from Cameroon and Columbia have a very different Al dynamics related to their differences in contents of amorphous Al and their CEC.
The breeding programmes at EMBRAPA-CNPMS, Brasil, IRAD, Cameroun, and INRA, Guadeloupe identified germplasm with improved adaptation to soil acidity. Breeding efforts focussed on the selection of maize genotypes both Al-resistant and P-efficient. In a joint breeding effort tropical maize populations from different genetic origins and covering a wide range of adaptation to acid-soils abilities were crossed and evaluated in Guadeloupe, Cameroon, and Colombia. Results assessed genetic progress for adaptation to soil acidity.
Germplasm was characterised for Al resistance using different quick screening methods, root elongation in nutrient solution (CIRAD), hematoxilyn staining (UAB, EMBRAPA), and Al-induced callose formation (UH) with fairly comparable classifications. A non-destructive technique allowing the testing of single seedlings for Al-induced callose formation and subsequently grain-yield formation on acid soil was developed by UHin co-operation with CIMMYT, Cali.
The physiological studies by EMBRAPA-CNPMS revealed that more than one mechanism is involved in Al resistance in maize. The results demonstrated that one of the traits associated with P acquisition efficiency is greater root growth of more P-efficient genotypes at low P supply. An analysis of root elongation under Al-stress by the UAB revealed clear differences in the time-dependence of the responses of Al-sensitive and Al-resistant maize cultivars which were accompanied by differences in the accumulation and compartmentation of Al in the root tips. The higher concentration of bound polyamines in Al-resistant cultivars is in line with the idea of a protective role of polyamines against Al. UH clarified that in maize an Al avoidance mechanism does not interfere with the assessment of genotypic Al resistance by uniform application of Al in hydroculture. Al supply rather than P deficiency clearly enhanced the release of particularly and specifically citrate in Al-resistant cultivars. Using a sandwich technique 10 cultivars differed widely in their P mobilisation capacity. No relationship was obvious between the reported adaptation of the cultivars to acid, Al-toxic soils with low P availability. The role of cell-wall properties, pectin content and its degree of methylation in modulating Al resistance was shown using intact plants, cell-suspension cultures, and PME transgenics.
In the area of agronomy CORPOICA, Colombia, and IRAD, Cameroun, conducted complex long-term field experiments with the objective to evaluate the long-term implications of strategies for improving maize production on acid soils. The results clearly show that at both locations adapted cultivars and organic amendments can at least partly substitute for lime application, but high productivity and sustainabilty can only be achieved by both genetic and agronomic strategies. The evaluation of the root growth studies conducted in close co-operation between CORPOICA and CIRAD revealed that root-length densities in the soil were mainly affected by application of organic matter. The software package "RACINE" has been improved to calculate root-length densities from root counts on a vertical transversal plane.
In the area of plant-soil relationships CIRAD in co-operation with INRA-UFR, Monpellier, developed a soil ionic exchange model using a simplified system and calibrated it using soil from the field experimental sites in Cameroon and Colombia. The results highlight the importance of the processes of dissolution and diffusion of Al in the rhizosphere and the necessity to take into consideration Al-complexing compounds such as organic acids.
In conclusion, the better understanding of the inheritance and of the physiological mechanism leading to acid soil tolerance in general and Al resistance and P efficiency as principal requirements for adaptation to such soils, specifically, lead to the development of quick screening techniques and allowed the better focus of molecular approaches thus facilitating and enhancing the progress made by breeding. Consequently, new varieties with improved yields on acid soils have been released in Brasil and Cameroon.